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The Minié ball was a conical- soft lead bullet, slightly smaller than the intended firearm barrel's bore, with (originally) four exterior grease-filled grooves and a conical hollow in its base. The bullet was designed by Minié with a small iron plug and a lead skirting. Its intended purpose was to expand under the pressure and obturate the barrel and increase muzzle velocity.

The precursor to the Minié ball was created in 1848 by the French Army captains Montgomery and Henri-Gustave Delvigne. Their design was made to allow rapid muzzle loading of rifles, an innovation that brought about the widespread use of the rifle rather than the smoothbore musket as a mass battlefield weapon. Delvigne had invented a ball that could expand upon ramming to fit the grooves of a rifle in 1826.[1] The design of the ball had been proposed in 1832 as the cylindro-conoidal bullet by Captain John Norton,[2] but had not been adopted.

The bullet could be quickly removed from the paper cartridge with the gunpowder poured down the barrel and the bullet pressed past the muzzle rifling and any detritus from prior shots. It was then rammed home with the ramrod, which ensured that the charge was packed and the hollow base was filled with powder. When the rifle was fired, the expanding gas pushed forcibly on the base of the bullet, deforming it to engage the rifling. This provided spin for accuracy, a better seal for consistent velocity and longer range, and cleaning of barrel detritus.

Wounds inflicted by the conical Minié ball were different from those caused by the round balls from smoothbore muskets, since the conical ball had a higher muzzle velocity and greater weight. Round balls tended to remain lodged in the flesh, and they were often observed to take a winding path through the body. Flexed muscles and tendons, as well as bone, could cause the round ball to deviate from a straight path. The Minié ball tended to cut a straight path and usually went all the way through the injured part; the ball seldom remained lodged in the body. If a Minié ball struck a bone, it usually caused the bone to shatter. The damage to bones was usually severe enough to necessitate amputation.[3]